The present invention relates to a lead-tolerant catalyst system and method for oxidizing exhaust gas containing lead compounds prior to discharging the exhaust gas into the atmosphere. The catalyst system of the present invention exhibits enhanced stability as a result of resistance to lead poisoning when effecting the conversion of auto exhaust gases or other lead-containing exhaust gas streams of a noxious nature.
Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. In recent years, with the ever growing number of automobiles powered by internal combustion engines, the discharge of pollutants therefrom has been of increasing concern, particularly in urban areas where the problem is more acute, and the control thereof has become exceedingly important. Of the various methods which have been proposed for converting carbon monoxide, hydrocarbon and nitrogen oxide pollutants to innocuous products, the incorporation of a catalyst converter in the exhaust system of an internal combustion engine holds the most promise of meeting the increasingly rigid standards established for automotive vehicles by the responsible governmental authorities.
In order to achieve the conversion of carbon monoxide in hydrocarbon pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ratio control means which function in response to a feedback signal from an oxygen sensor in the engine exhaust system. The air-to-fuel ratio control means is typically programmed to provide fuel and air to the engine at a ratio conducive to a near stoichiometric balance of oxidants and reductants in the hot exhaust gases at engine cruising conditions, and to a stoichiometric excess of reductants at engine idling and acceleration conditions. The result is that the composition of the exhaust gases with which the catalyst is contacted fluctuates almost constantly, such that conditions to which the catalyst is exposed are alternatively netreducing and net-oxidizing. A catalyst for the oxidation of carbon monoxide and hydrocarbons must be capable of operating in such a dynamic environment.
Aside from problems of operating environment, exhaust gas oxidation catalysts are often subject to contamination by contact with poisoning constituents which act to decrease the catalyst's ability to convert noxious components. For example, it has become common practice to add to automotive gasolines certain anti-knock compounds. These compounds often act as catalyst poisons for automotive exhaust oxidation catalysts employed for the conversion of hydrocarbons, and CO in automotive exhaust gases. Typical of the anti-knock compounds which pose the greatest potential for poisoning exhaust catalysts are alkyl lead compounds. Lead compounds which are the combustion products of the alkyl lead anti-knock component exit the engine with the exhaust gas. When the lead containing exhaust gas is contacted with the exhaust gas catalyst, lead is deposited on the catalytically active sites of the catalyst thereby causing deactivation. After long exposure to exhaust gases containing lead compounds the exhaust gas catalyst becomes substantially inactive and incapable of performing its intended functions.
Two alternative solutions exist for coping with this poisoning of exhaust gas catalyst. The first alternative is to eliminate the poisons from the exhaust prior to contact with the catalyst. In the case of lead compounds in automobile exhaust gases this has been achieved by eliminating alkyl lead compounds as anti-knock components of gasoline. Unfortunately, the elimination of lead-based anti-knock compounds has resulted in an overall increase in the cost of gasoline. This is because the displacement of lead anti-knock compounds has been compensated for by the additional blending of high octane components into gasoline in order to meet octane requirements. Producing high octane components such as benzene, toluene, and xylenes requires expensive and elaborate refining techniques, typically catalytic reforming. All in all these added steps act to increase the cost of gasoline. A second and more desirable alternative to removing lead-based anti-knock compounds would be to develop a lead-tolerant exhaust gas oxidation catalyst. Such a catalyst should be able to maintain high exhaust gas oxidation activity despite long term exposure to exhaust gases containing lead compounds. Such a catalyst would allow continued use of lead-containing anti-knock compounds thereby reducing the cost of gasoline while assuring the continued viable oxidation of the noxious exhaust gas components.